Simple ladle refining method

ABSTRACT

The present invention provides a simplified ladle refining process capable of efficiently heating a molten steel in a short period of time while scattering and adhesion of splashes and erosion of refractories are suppressed. A simplified ladle refining process for refining a molten steel in a ladle comprising inserting an immersion snorkel into a ladle and blowing an oxidizing gas onto the surface of a molten steel within the immersion snorkel through a lance while the molten steel is being stirred by blowing an inert gas through the bottom of the ladle, wherein the lance has a ratio (d 0 /d t ) of a nozzle outlet diameter d 0  (mm) to a nozzle throat diameter d t  (mm) of from 1.2α to 2.5α wherein α is defined as a function of a back pressure.

TECHNICAL FIELD

The present invention relates to a simplified process for refining amolten steel previously refined in a refining furnace, and particularlyto a process for efficiently heating a molten steel.

BACKGROUND ART

It has been widely known that, in a steelmaking process, at the time ofsupplying a molten steel primary-refined by a converter or an electricfurnace to a casting process represented by continuous casting, themolten steel is treated in advance by a simplified secondary refiningapparatus for the purpose of adjusting the chemical composition andtemperature.

Japanese Unexamined Patent Publication (Kokai) No. 53-149826 discloses aprocess for heating a molten steel comprising immersing an immersionsnorkel in a molten steel to provide a protective wall while the moltensteel is being stirred by blowing a gas through a gas-blowing nozzle inthe bottom of a ladle, and blowing oxygen gas onto the molten steelthrough an oxygen-blowing pipe while an oxidation reaction agent isbeing added to a region surrounded by the protective wall through asupply snorkel. Moreover, Japanese Unexamined Patent Publication (Kokai)No. 61-235506 discloses a process for refining a molten steel in a ladlecomprising blowing an oxidizing gas through a lance onto the surface ofa molten steel within an immersion snorkel inserted into a ladle, whilethe molten steel is stirred by blowing an inert gas through the bottomof the ladle, wherein prior to blowing the oxidizing gas through atop-blowing lance, an oxidation reaction agent is added within theimmersion snorkel, and then blowing the oxidizing gas and adding theoxidation reaction agent are continuously conducted through thetop-blowing lance.

However, although the structure of the top-blowing lance and theconditions for blowing oxygen significantly influence the efficiency forheating a molten steel, neither the influences nor the conditions forefficiently heating a molten steel are disclosed in the patentpublication. Japanese Unexamined Patent Publication (Kokai) No.61-129264 discloses a process for refining a molten steel in a ladlecomprising inserting an immersion snorkel into a ladle while a moltensteel is being stirred by blowing an inert gas through the bottom of theladle, and blowing an oxidizing gas onto the molten steel surface withinthe immersion snorkel through a lance, wherein an annular lancecomprising an inner pipe and an outer pipe is used as the lancementioned above, oxygen and an inert gas are supplied to the inner pipeand the outer pipe, respectively, while the blowing pressure P1 of theoxidizing gas from the inner pipe is being made lower than the blowingpressure P2 of the inert gas from the outer pipe (P1<P2), and anoxidation reaction agent is added within the immersion snorkel inaccordance with the conditions for blowing oxygen.

However, the process has the problem of a high gas cost because a largeamount of a costly inert gas is used therein. On the other hand,Sumitomo Kinzoku, vol. 45-3, page 66 and the following pages (1993)discloses an embodiment of using a castable annular pipe as an oxygenlance. However, since a lance having a castable structure lacksdurability, the lance has the disadvantage of being costly.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a simplified refiningprocess capable of efficiently heating a molten steel in a short periodof time while the scattering and adhesion of splashes and the erosion ofrefractories are suppressed.

In order to achieve the object mentioned above, the present inventionprovides a process as described below.

(1) A simplified ladle refining process for refining a molten steel in aladle comprising inserting an immersion snorkel into a ladle and blowingan oxidizing gas onto the surface of a molten steel within the immersionsnorkel through a lance while the molten steel is being stirred byblowing an inert gas through the bottom of the ladle, wherein the lancehas a ratio (d₀/d_(t)) of a nozzle outlet diameter d₀ (mm) to a nozzlethroat diameter d_(t) (mm) of from 1.2α to 2.5α wherein α is calculatedby the formula (1):

α=[(1/M·{(1+0.2×M ²)/1.2}³]^(½)  (1)

wherein M={5×(P^({fraction (2/7)})−1)}^(½) wherein P is a back pressure(kgf/cm², absolute pressure).

The oxygen-blowing lance desirably has a water-cooled structure.Moreover, an alloy containing Al and Si is added to the molten steelduring blowing oxygen so that the molten steel has an Al content of1.6×S−1.9×S kg/t or a Si content of 1.25×S−1.5×S kg/t wherein S (Nm³/t)is a unit requirement of oxygen, and the heat produced by such oxidationreactions is desirably utilized.

(2) The simplified ladle refining process according to (1), wherein theratio (L/a) of a cavity depth L (mm) of the molten steel surfacecalculated by the formulas (2) to (8) to a fire spot diameter a (mm)calculated by the formulas (3) and (9) is defined to be from 0.5 to0.005:

0.016×L ^(½) =H _(c)/(LH+L)  (2)

wherein LH is a distance (lance gap, mm) between the lance and themolten steel surface, and H_(c) is a jet core length (mm) calculated bythe formula (3):

H _(c) =f×M _(op)×(4.2+1.1×M _(op) ²)×dt  (3)

wherein M_(op) depends on the lance shape and is obtained by solving theformula (4):

d ₀ /d _(t)=[(1/M _(op))×{(1+0.2×M _(op) ²)/1.2}³]^(½)  (4)

and f is calculated by the formula (5) or (6):

f=0.8X−0.06 (X<0.7)  (5)

f=−2.7X ⁴+17.7X ³−41X ²+40X−13(X>0.7)  (6)

wherein X=P_(o)/P_(op) wherein P_(op) is calculated by the formula (7)using M_(op), and P_(o) is calculated by the formula (8):

P _(op)={(M _(op) ²/5)+1}^({fraction (7/2)})  (7)

P _(o) =F/(0.456×n×d _(t) ²)  (8)

wherein F is an oxygen supply rate (Nm³/hr), and n is a number ofnozzles, and

a=0.425×(LH−Hc)+d _(t)  (9)

(3) The simplified ladle refining process according to (1) or (2),wherein the ratio (D/a) of an immersion snorkel diameter D (mm) to thefire spot diameter a (mm) is defined to be from 1.5 to 8.

(4) The simplified ladle refining process according to any one of (1) to(3), wherein the ratio (LH/a) of the lance gap LH (mm) to the fire spotdiameter a (mm) is defined to be from 2 to 3.5.

In the process of the present invention, an oxidation reaction agent isadded to a molten steel within the immersion snorkel while the interiorof the immersion snorkel is being maintained at the atmosphericpressure, and the molten steel can be heated by oxidizing the oxidationreaction agent.

Alternatively, the molten steel can be heated by oxidizing an oxidationreaction agent having been added to the molten steel in advance whilethe interior of the immersion snorkel is being maintained at theatmospheric pressure.

Furthermore, prior to or subsequent to heating the molten steel byoxidizing the oxidation reaction agent under the atmospheric pressure asdescribed above, the molten steel can be decarburized by blowing anoxidizing gas onto the molten steel surface within the immersion snorkelthrough the lance while the interior of the immersion snorkel isevacuated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the fundamental construction of anapparatus for carrying out the process of the present invention.

FIG. 2 is a graph showing the relationship between d₀/d_(t) and aheating efficiency (η).

FIG. 3 is a graph showing the relationship between L/a and a heatingefficiency (η).

FIG. 4 is a graph showing the relationship between D/a and a heatingefficiency (η).

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically shows the fundamental construction of an apparatusfor carrying out the process of the present invention.

The apparatus in FIG. 1 fundamentally comprises a ladle 1, an immersionsnorkel 2 (inside diameter D) and a top-blowing lance 3. The ladle 1 isequipped with a bottom-blowing tuyere (porous bricks) in the bottom. Amolten steel 5 from a converter etc., is placed in the ladle 1, and thelower end of the immersion snorkel 2 is immersed in the molten steel 5from the top. An inert gas such as Ar is blown into the molten steel inthe immersion snorkel 2 through the bottom blowing tuyere 4 to form aplume region 6 in the molten steel 5 and stir the molten steel. Anoxidizing gas such as oxygen is blown onto the molten steel surfacewhich is being stirred through a top-blowing lance 3 inserted into theimmersion snorkel 2. The lance 3 has a nozzle outlet diameter d₀ and anozzle throat diameter d_(t). The oxidizing gas such as oxygen blownthrough the lance 3 forms a jet core 7, and strikes the surface of themolten steel 5, thereby forming a cavity having a depth L and a diameter(fire spot diameter) a on the surface of the molten steel 5. The lancegap is defined by a distance LH from the lower end of the lance 3 to thesurface of the molten steel 5.

In order to heat the molten steel most efficiently, the presentinventors have found that the requirements explained below must besatisfied.

I: Scattering of molten steel particles (splashes) produced by thecollision energy of the top-blown gas on the molten steel surface mustbe decreased. That is, since the scattered particles are produced fromthe surface (fire spot) of collision between the molten steel having thehighest temperature and oxygen, the particles have a temperature higherthan that of the bulk of the molten steel. However, since the moltensteel particles at high temperature are scattered, the particles releasea sensible heat in the space during scattering to raise the exhaust gastemperature. Moreover, the particles themselves act, on the contrary, tocool the molten steel because the particles fall on the molten steelwhile having a lowered temperature. Accordingly, formation of splashescauses the exhaust gas temperature to rise and the heating efficiency ofthe molten steel to fall.

II: The top-blown gas blown through the lance is blown onto the moltensteel surface where the inert gas bubbles blown through the bottom ofthe ladle float. A fresh molten steel surface is always exposed in thebubble breaking region (plume region) of the bottom-blown gas bubbles onthe molten steel surface. Accordingly, the oxidation reactions of Al andSi proceed very efficiently, and the heating efficiency can be enhancedby blowing an oxygen gas onto the region. Particularly when oxygen issupplied by a “soft blow” so that splashes are not made, efficientheating cannot be effected so long as oxygen is not blown onto the plumeregion.

If oxygen is blown onto a surface portion other than the plume region, afilm of oxide such as Al₂O₃ or SiO₂ will be stably formed. The oxidefilm has poor thermal conductivity, and hinders the heat transfer.Furthermore, even when oxygen is concentratedly supplied to a localportion of the plume region, a film of oxide such as Al₂O₃ or SiO₂ isstably formed in the surface portion, and hinders heat transfer.

The invention (1) defines the lance design in order to realize the lancefactor I. That is, a high heating efficiency as shown in FIG. 2 can beobtained by defining the ratio (d₀/d_(t)) of a nozzle outlet diameter d₀(mm) to a nozzle throat diameter d_(t) (mm) to be from 1.2α to 2.5α.Herein, the heating efficiency (η, %) is a ratio of a heating amountactually measured to a theoretical heating amount calculated on theassumption that all the oxygen blown onto the molten steel reacts withAl (formula (10)):

η=100×0.21×ΔT×1000/(7420×W _(Al))  (10)

The calculated a obtained from the formula (1) corresponds to the ratio(d₀/d_(t))_(op) of a nozzle mouth diameter to a nozzle throat diameterwhich gives a proper expansion condition when the back pressure is P,and (d₀/d_(t))/α is a parameter showing the magnitude of a shift fromthe proper expansion conditions. That (d₀/d_(t))/α is at least 1signifies that the nozzle outlet diameter is excessively widenedcompared with one under proper expansion conditions, that is, the nozzleoutlet diameter is under excessive expansion conditions. When the nozzleoutlet diameter has an excessive expansion, a pressure loss is producedwithin the nozzle, and the jet becomes a soft blow. However, when(d₀/d_(t)) is less than 1.2α, the effect of soft blowing cannot beobtained due to an insufficient degree of excessive expansion. When(d₀/d_(t)) is larger than 2.5α, the flow rate of the blown gas at thenozzle tip becomes excessively low. As a result, the base metal and slagscattered and directed against the nozzle invade the interior of thenozzle to shorten the nozzle life.

The oxidizing gas may either be 100% oxygen or contain up to 50% ofnitrogen, Ar or the like.

The invention (2) shows more proper heating conditions concerning thefactors I and II. That is, a still higher heating efficiency is obtainedby defining the ratio (L/a) of a cavity depth L (mm) of the molten steelsurface to a fire spot diameter a (mm) to be from 0.5 to 0.005, as shownin FIG. 3. Herein, the formulas (2) to (9) for calculating L and a havebeen experimentally obtained by the present inventors. When L/a islarge, soft blown oxygen comes to strike the molten steel surface in awide region.

The plume region of bottom-blown bubbles is spread to such an extentthat the region substantially covers the molten steel surface within theimmersion snorkel. Accordingly, when L/a is large, the heatingefficiency becomes extremely high because oxygen can be supplied to theplume region in a wide range without making splashes. When L/a is largerthan 0.5 (log(L/a)>−0.3), the heating efficiency significantly lowersdue to the formation of splashes. When L/a is smaller than 0.005(log(L/a)<−2.3), splashes occur to a small extent. However, thetop-blown jet is too weak, and the so-called ineffective oxygen, whichdoes not reach the molten steel surface, increases thereby lowering theheating efficiency.

The invention (3) defines the factor II. That is, since the oxygen gasis supplied while broadly covering the plume region by making D/a small,the heat input to the molten steel becomes extremely high. As shown inFIG. 4, when D/a is larger than 8, oxygen is concentratedly supplied toa local portion in the plume region. As a result, a film of oxides suchas Al₂O₃ and SiO₂ is stably formed to lower the heating efficiency. Asmaller D/a gives better results because the molten steel surface withinthe immersion snorkel becomes an almost complete plume region. However,when D/a is smaller than 1.5, the fire spot approaches the immersionsnorkel wall too closely, and erosion of the refractories becomesexcessive.

The invention (4) defines the lance gap in order to increase the heatingefficiency. As shown in Table 1, when LH/a is smaller than 2, the lanceapproaches the molten steel surface too closely. As a result, waving ofthe molten steel surface caused by the bottom-blown gas erodes thelance. When LH/a is larger than 3.5, the free jet zone of the oxygen gasbecomes too long. As a result, erosion of the immersion snorkelrefractories becomes significant under the influence of the radiation.

TABLE 1 Situation of erosion of Situation of Heat input immersionsnorkel erosion efficiency LH/a refractories of lance (%) Present 1 2.2No Slight 91 invention Present 2 2.8 No No 95 invention Present 3 3.1 NoNo 93 invention Present 4 3.4 Slight No 92 invention Comparative 5 1.9No Yes 90 Example Comparative 6 1.8 No Yes 93 Example Comparative 7 3.7Yes No 92 Example Comparative 8 4.0 Yes No 94 Example

EXAMPLE 1

Ladle refining was conducted according to the present invention underthe following conditions.

The weight W of a molten steel and the inside diameter of an immersionsnorkel were 350 ton and 1.5 m, respectively. Ar was used as abottom-blown gas. Ar was blown through porous bricks placed in thebottom of a ladle furnace at a flow rate of about 400 Nl/min.

Al was added to an Al-Si killed steel in the ladle at a rate of 80kg/min while oxygen gas was being blown thereonto at a rate of 3,000Nm³/hr through a top-blowing lance. A single annular nozzle water-cooledlance having a nozzle throat diameter (d_(t)) of 20.5 mm and a nozzleoutlet diameter (d₀) of 56 mm was used as the top-blowing lance. Theback pressure (P) was 15.65 kgf/cm² (absolute pressure). Since M and αat the back pressure were calculated to be 2.44 and 1.58, respectively,(d₀/d_(t)) was 2.11×α.

Furthermore, M_(op), P_(op), P₀ and f of the lance under properexpansion conditions were calculated to be 4, 156.8, 15.65 kgf/cm² and0.02, respectively. When H_(c) and the lance gap were 35.48 mm and 1,000mm, respectively, a was 430.52 mm, and L was 4.9 mm. Accordingly, L/a,D/a and LH/a were 0.011, 3.46 and 2.32, respectively. Oxygen blowing for7 minutes could heat the molten steel from 1,615 to 1,667° C., and theheating efficiency was 92%. Splashes were small, and the erosion of therefractories was slight.

COMPARATIVE EXAMPLE

The same apparatus as in Example 1 was used in the present ComparativeExample.

A single annular nozzle water-cooled lance having a nozzle throatdiameter d_(t) of 20.5 mm and a nozzle outlet diameter d₀ of 34.25 mmwas used as the top-blowing lance. The back pressure P was 15.65 kgf/cm²(absolute pressure). Since M and α at the back pressure were calculatedto be 2.44 and 1.58, respectively, (d₀/d_(t)) was 1.06×α. Furthermore,M_(op), P_(op), P₀ and f of the lance under proper expansion conditionswere calculated to be 2.55, 19.1, 15.65 kgf/cm² and 0.77, respectively.When H_(c) and the lance gap were 663.46 mm and 1,000 mm, respectively,a was 163.5 mm, and L was 640 mm. Accordingly, L/a, D/a and LH/a were3.91, 9.11 and 6.11, respectively. Oxygen blowing for 7 minutes couldheat the molten steel from 1,605 to 1,645° C., and the heatingefficiency was 71%. Splashes were large, and erosion of the refractorieswas observed due to the rise of the exhaust gas temperature.

EXAMPLE 2

The same refining apparatus as in Example 1 was used. An oxygen gas wasblown through the top-blowing lance onto a molten steel, the C contentof which had been lowered to 0.09% by a converter, and Al wassimultaneously added in the same manner as in Example 1 to heat themolten steel to 1,654° C. with a heating efficiency of 94%. The interiorof the immersion snorkel was consecutively evacuated to have a degree ofvacuum of 250 to 350 Torr. Oxygen gas was blown onto the molten steel ata rate of 3,000 Nm³/hr using the same lance to lower the C content from0.09 to 0.05%. The lance gap was then from 1,000 to 1,500 mm, and theflow rate of the bottom-blown Ar was 300 to 500 Nl/min. Afterdecarburization, the inner pressure of the immersion snorkel wasrestored to the atmospheric pressure, and the molten steel wasdeoxidized by adding Al thereto.

EXAMPLE 3

The same refining apparatus as in Example 1 was used. A non-deoxidizedmolten steel the C content of which had been lowered to 0.09% by aconverter and to which Al had not been added was placed in theapparatus. The interior of the immersion snorkel was evacuated to have adegree of vacuum of 250 to 350 Torr. Oxygen gas was blown through thesame lance as in example 1 at a rate of 3,000 Nm³/hr onto the moltensteel to lower the carbon content from 0.09% to 0.05%. The lance gap wasthen from 1,000 to 1,500 mm, and the flow rate of the bottom-blown Arwas from 300 to 500 Nl/min. After decarburization, the inner pressure ofthe immersion snorkel was restored to the atmospheric pressure, oxygengas was blown onto the molten steel through the top-blowing lance in thesame manner as in Example 1, and Al was simultaneously added thereto toheat the molten steel. Al was subsequently added thereto to make the Alcontent 0.025% and to deoxidize the molten steel.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, a molten steelcan be efficiently heated in a short period of time while scattering andadhesion of splashes and erosion of refractories are suppressed.

What is claimed is:
 1. A simplified ladle refining process for refininga molten steel in a ladle comprising inserting an immersion snorkel intoa ladle and blowing an oxidizing gas onto the surface of a molten steelwithin the immersion snorkel through a lance while the molten steel isbeing stirred by blowing an inert gas through the bottom of the ladle,wherein the lance has a ratio (d₀/d_(t)) of a nozzle outlet diameter d₀(mm) to a nozzle throat diameter d_(t) (mm) of from 1.2α to 2.5α whereinα is calculated by the formula (1): α=[(1/M·{(1+0.2×M ²)/1.2}³]^(½)  (1)wherein M={5×(P^({fraction (2/7)})−1)}^(½) wherein P is a back pressure(kgf/cm², absolute pressure).
 2. The simplified ladle refining processaccording to claim 1, wherein the ratio (L/a) of a cavity depth L (mm)of the molten steel surface calculated by the formulas (2) to (8) to afire spot diameter a (mm) calculated by the formulas (3) and (9) isdefined to be from 0.5 to 0.005: 0.016×L ^(½) =H _(c)/(LH+L)  (2)wherein LH is a distance (lance gap, mm) between the lance and themolten steel surface, and H_(c) is a jet core length (mm) calculated bythe formula (3): H _(c) =f×M _(op)×(4.2+1.1×M _(op) ²)×d _(t)  (3)wherein M_(op) depends on the lance shape and is obtained by solving theformula (4): d ₀ /d _(t)=[(1/M _(op))×{(1+0.2×M _(op) ²)/1.2}³]^(½)  (4)and f is calculated by the formula (5) or (6): f=0.8X−0.06 (X<0.7)  (5)f=−2.7X ⁴+17.7X ³−41X ²+40X−13(X>0.7)  (6) wherein X=P_(o)/P_(op)wherein P_(op) is calculated by the formula (7) using M_(op), and P_(o)is calculated by the formula (8): P _(op)={(M _(op)²/5)+1}^({fraction (7/2)})  (7) P _(o) =F/(0.456×n×d _(t) ²)  (8)wherein F is an oxygen supply rate (Nm³/hr), and n is a number ofnozzles, and a=0.425×(LH−Hc)+d_(t)  (9).
 3. The simplified ladlerefining process according to claim 1, wherein the ratio (D/a) of animmersion snorkel diameter D (mm) to the fire spot diameter a (mm) isdefined to be from 1.5 to
 8. 4. The simplified ladle refining processaccording to claim 1, wherein the ratio (LH/a) of the lance gap LH (mm)to the fire spot diameter a (mm) is defined to be from 2 to 3.5.
 5. Thesimplified ladle refining process according to claim 1, wherein anoxidation reaction agent is added to the molten steel within theimmersion snorkel, while the interior of the immersion snorkel is beingmaintained at the atmospheric pressure, to be oxidized and heat themolten steel.
 6. The simplified ladle refining process according toclaim 1, wherein an oxidation reaction agent having been added inadvance to the molten steel is oxidized, while the interior of theimmersion snorkel is being maintained at the atmospheric pressure, toheat the molten steel.
 7. The simplified ladle refining processaccording to claim 5, wherein prior to or subsequently to heating themolten steel by oxidizing the oxidation reaction agent under theatmospheric pressure, an oxidizing gas is blown onto the molten steelsurface within the immersion snorkel, through the lance, to decarburizethe molten steel.